Email marketing is a popular and effective digital marketing tool, with over 90 trillion emails sent annually. Subject lines are important for piquing interest and driving email opens. Testing different subject lines and content with A/B testing helps determine what's most effective. Email service providers host email marketing services and handle sending large volumes of emails while providing reporting on campaign performance. Laws like CAN-SPAM establish rules for commercial emails regarding opt-outs, header information, and penalties for violations.